如何在 VB.NET 查询中使用 MySQL 变量？

Question

When using a query with MySQL variables from VB.NET I get an error

Fatal error encountered during command execution.

The problem is when trying to use a MySQL variable like @sum in a VB.NET string, this variable is recognized as a parameter of the MySqlCommand and not as part of the string itself.

Here is the code that I use.

dim query as string = "SET @sum = 0; SELECT @sum := (@sum + debe - haber) AS 'Saldo' from cccustomers WHERE customer = 443"
using con
    If con.State = ConnectionState.Closed Then con.Open()
    Using cmd As New MySqlCommand(query, con)
        Dim adp As New MySqlDataAdapter(cmd)
        Dim dt As New DataTable
        adp.Fill(dt)
        DataGridView1.DataSource = dt
    end using
end using

I also tried this with exactly the same result:

dim sums as string = "@sum"
dim query as string = "SET " & sums & " = 0; SELECT " & sums & " := (" & sums & " + debe - haber) AS 'Saldo' from cccustomers WHERE customer = 443"

And I also tried this:

dim query as string = "SET @sum = 0; SELECT @sum := (@sum + debe - haber) AS 'Saldo' from cccustomers WHERE customer = 443"
using con
    If con.State = ConnectionState.Closed Then con.Open()
    Using cmd As New MySqlCommand(query, con)
        cmd.Parameters.AddWithValue("@sum", "@sum")
        Dim adp As New MySqlDataAdapter(cmd)
        Dim dt As New DataTable
        adp.Fill(dt)
        DataGridView1.DataSource = dt
    end using
end using

But in this case, the problem is that it passes me the variable as a string with quotes '@sum' and that gives a different error in the query.

If I make that same query directly in phpmyadmin it works correctly, the problem I have is: How can I use the MySQL variable in VB.net?

I need the variable because in each line I have to show the accumulated balance and not only show the balance of each particular line.

Thanks for your time.

EDIT: Table Definition, Sample Data and expected Output

CREATE TABLE `stack` (
  `id` int(11) NOT NULL,
  `debe` double NOT NULL,
  `haber` double NOT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

--
-- Volcado de datos para la tabla `stack`
--

INSERT INTO `stack` (`id`, `debe`, `haber`) VALUES
(1, 100, 0),
(2, 100, 0),
(3, 100, 0),
(4, 100, 0),
(5, 0, 50),
(6, 100, 0),
(7, 0, 150),
(8, 100, 0),
(9, 100, 0),
(10, 100, 0),
(11, 0, 100),
(12, 100, 0);

--
-- Índices para tablas volcadas
--

--
-- Indices de la tabla `stack`
--
ALTER TABLE `stack`
  ADD PRIMARY KEY (`id`);

--
-- AUTO_INCREMENT de las tablas volcadas
--

--
-- AUTO_INCREMENT de la tabla `stack`
--
ALTER TABLE `stack`
  MODIFY `id` int(11) NOT NULL AUTO_INCREMENT, AUTO_INCREMENT=13;
COMMIT;

The Query: (works on PhpMyadmin but NOT WORK on VB.NET because the symbol @ in MySqlCommand expects a parameter - I don't want to send a parameter, I want to Send a MySql variable )

SET @sum = 0; 
SELECT debe, haber, @sum := (@sum + debe - haber) AS 'Saldo' from stack;

Expected output:

Code on VB.NET with the SAME QUERY:

dim query as string = "SET @sum = 0; SELECT debe, haber, @sum := (@sum + debe - haber) AS 'Saldo' from stack;"
using con
    If con.State = ConnectionState.Closed Then con.Open()
    Using cmd As New MySqlCommand(query, con)
        Dim adp As New MySqlDataAdapter(cmd)
        Dim dt As New DataTable
        adp.Fill(dt)
        DataGridView1.DataSource = dt
    end using
end using

Result on VB.NET

This exception means that @sum is expected to be defined as a parameter , but I don't want to define a parameter, I want to declare a MySQL variable .

Answer 1

To use MySQL variables in a VB.NET query, you'll need to use the MySqlCommand class and its Parameters property. Here's an example of how to do it:

Dim conn As New MySqlConnection("connection string")
Dim cmd As New MySqlCommand("SELECT * FROM table WHERE column = @param1 AND 
column2 = @param2", conn)

cmd.Parameters.AddWithValue("@param1", "value1")
cmd.Parameters.AddWithValue("@param2", "value2")

conn.Open()
Dim reader As MySqlDataReader = cmd.ExecuteReader()

' Process the data returned by the query
While reader.Read()
  ' Access column values using reader("column_name")
End While

reader.Close()
conn.Close()

In this example, we create a MySqlCommand object and specify a SQL query with placeholders for the variables (@param1 and @param2). Then, we use the Parameters property to add values for the variables using the AddWithValue() method.

When the query is executed, the placeholders are replaced with the actual values specified in the Parameters collection. This allows you to use variables in your queries in a safe and efficient way, as the values are automatically escaped and properly formatted.